Semiconductor devices with low dislocation defects

ABSTRACT

Semiconductor devices having a low density of dislocation defects can be formed of epitaxial layers grown on defective or misfit substrates by making the thickness of the epitaxial layer sufficiently large in comparison to the maximum lateral dimension. With sufficient thickness, threading dislocations arising from the interface will exit the sides of the epitaxial structure and not reach the upper surface. Using this approach, one can fabricate integral gallium arsenide on silicon optoelectronic devices and parallel processing circuits. One can also improve the yield of lasers and photodetectors.

This is a division of application Ser. No. 07/561744, filed Aug. 2,1990, now U.S. Pat. No. 5,158,907.

FIELD OF THE INVENTION

This invention relates to semiconductor devices having a low density ofdislocation defects and, in particular, to semiconductor devicescomprising limited area epitaxial regions grown on either misfitsubstrates or substrates having a high density of dislocation defects.It further concerns methods for making and using such devices.

BACKGROUND OF THE INVENTION

A low level of dislocation defects is important in a wide variety ofsemiconductor devices and processes. Dislocation defects partition anotherwise monolithic crystal structure and introduce unwanted and abruptchanges in electrical and optical properties. Dislocation defects canarise in efforts to epitaxially grow one kind of crystalline material ona substrate of a different kind of material (heterostructures) due todifferent crystalline lattice sizes of the two materials. Misfitdislocations form at the mismatched interface to relieve the misfitstrain. Many misfit dislocations have vertical components, termedthreading segments, which terminate at the surface. These threadingsegments continue through all subsequent layers added. Dislocationdefects can also arise in the epitaxial growth of the same material asthe substrate (homostructures) where the substrate itself containsdislocations. Some of the dislocations replicate as threadingdislocations in the epitaxially grown material. Such dislocations in theactive regions of semiconductor devices such as diodes, lasers andtransistors, seriously degrade performance.

To avoid dislocation problems, most semiconductor heterostructuredevices have been limited to semiconductor layers that have very closelylattice-matched crystal structures. Typically the lattice mismatch iswithin 0.1%. In such devices a thin layer is epitaxially grown on amildly lattice mismatched substrate. So long as the thickness of theepitaxial layer is kept below a critical thickness for defect formation,the substrate acts as a template for growth of the epitaxial layer whichelastically conforms to the substrate template. While lattice matchingand near matching eliminates dislocations in a number of structures,there are relatively few lattice-matched systems with large energy bandoffsets, limiting the design options for new devices.

There is considerable interest in heterostructure devices involvinggreater epitaxial layer thickness and greater lattice misfit thanpresent technology will allow. For example, it has long been recognizedthat gallium arsenide grown on silicon substrates would permit a varietyof new optoelectronic devices marrying the electronic processingtechnology of silicon VLSI circuits with the optical componenttechnology available in gallium arsenide. See, for example, Choi et al,"Monolithic Integration of Si MOSFET's and GaAs MESFET's", IEEE ElectronDevice Letters, Vol. EDL-7, No. 4, April 1986. Highly advantageousresults of such a marriage include high speed gallium arsenide circuitscombined with complex silicon VLSI circuits and gallium arsenideoptoelectronic interface units to replace wire interconnects betweensilicon VLSI circuits. Progress has been made in integrating galliumarsenide and silicon devices. See, for example, Choi et al, "MonolithicIntegration of GaAs/AlGaAs Double-Heterostructure LED's and Si MOSFET's"IEEE Electron Device Letters, Vol. EDL-7, No. 9, September 1986;Shichijo et al, "Co-Integration of GaAs MESFET and Si CMOS Circuits",IEEE Electron Device Letters, Vol. 9, No. 9, September 1988. However,despite the widely recognized potential advantages of such combinedstructures and substantial efforts to develop them, their practicalutility has been limited by high defect densities in gallium arsenidelayers grown on silicon substrates. See, for example, Choi et al,"Monolithic Integration of GaAs/AlGaAs LED and Si Driver Circuit", IEEEElectron Device Letters, Vol. 9, No. 10, October 1988 (p. 513). Thuswhile basic techniques are known for integrating gallium arsenide andsilicon devices, there exists a need for producing gallium arsenidelayers having a low density of dislocation defects.

There is also considerable interest in growing low defect densitygallium arsenide surfaces irrespective of the type of substrate. Galliumarsenide is prone to dislocation defects; and, as a consequence, devicesgrown on gallium arsenide substrates have a notoriously low yield.

SUMMARY OF THE INVENTION

In contrast with the prior art approach of minimizing dislocationdefects by limiting misfit epitaxial layers to less than a criticalthickness for elastic conformation to the substrate, the presentinvention utilizes greater thicknesses and limited lateral areas toproduce limited area regions having upper surfaces exhausted ofthreading dislocations. Since threading dislocations propagate with alateral as well as a vertical component, making the thicknesssufficiently large in comparison to the lateral dimension permits thethreading dislocations to exit the sides of the epitaxial structure. Theupper surface is thus left substantially free of defects. As a result,one can fabricate monolithic heterostructure devices, such as galliumarsenide on silicon optoelectronic devices, long sought in the art butheretofore impractical due to dislocation defects. As anotherembodiment, one can fabricate a monolithic structure using galliumarsenide circuitry to perform high speed processing tasks and siliconVLSI circuitry to perform complex, lower speed tasks. In yet anotherembodiment, one can fabricate arrays of low defect density devices on ahigh defect density substrate, substantially improving the yield.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail. In the drawings:

FIG. 1 is a schematic cross section illustrating the problem ofthreading dislocations addressed by the present invention.

FIG. 2 is a schematic cross section of a first embodiment of asemiconductor workpiece provided with limited area regions of low defectdensity in accordance with the invention.

FIG. 3 is a schematic cross section of a second embodiment of asemiconductor workpiece in accordance with the invention.

FIG. 4 is a schematic cross section of a third embodiment of theinvention;

FIG. 5 schematically illustrates the use of the invention to provideoptical input, optical interconnections and/or optical output tointegrated circuits in the substrate.

FIG. 6 schematically illustrates the use of the invention to provide asupplementary high speed circuit to an integrated circuit in thesubstrate; and

FIG. 7 schematically illustrates the use of the invention to providearrays of low defect density regions on a high defect density substrate.

It is to be understood that these drawings are for purposes ofillustrating the concepts of the invention and are not to scale. Similarstructural elements are denoted by the same reference numeralsthroughout the drawing.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 is a schematic cross sectionillustrating the problem of threading dislocations resulting fromefforts to epitaxially grow a blanket layer 10 of crystalline materialon a crystalline substrate 11. As illustrated, dislocation defects 12form at the interface 13 between layers 10 and 11. Many of these defects12 have not only horizontal portions 14, termed misfit segments, butalso portions with vertical components 15, termed threading segments.Such threading segments can also arise as continuations of pre-existingthreading segments 16 in the substrate 11.

FIG. 2 is a schematic cross section of a first embodiment of asemiconductor workpiece provided with limited area regions of low defectdensity in accordance with the invention. The workpiece comprises amonolithic semiconductor substrate 20 having a major surface 21 coveredwith an insulating layer 22. The insulating layer includes one or moreopenings 23, and grown within the openings on substrate 20, one or morelimited area epitaxial regions 24 of low defect density semiconductor.The substrate 20 and the low defect region 24 can be differentcrystalline semiconductors having lattice mismatch in excess of 0.2%.

Because of dislocations or pre-existing threading segments at theinterface between substrate 20 and grown regions 24, threading segments15 and 16 arise from the interface. However each epitaxial region 24 hasa thickness t sufficiently large as compared with its maximum lateralextent L that the threading segments exit the sides of regions 24 ratherthan reaching the upper surfaces 27. The ratio of t/L required to insureexit of threading segments arising from the interface depends on thecrystalline orientation of the substrate. A (100) substrate requires aratio of 1; a (111) substrate requires a ratio of √2, and a (110)substrate requires √3/3. Ratios of 50% of these values provide a usefullevel of defect elimination.

In a preferred embodiment, the substrate 20 is (100) monocrystallinesilicon, the insulating layer 22 is silicon oxide, and the limited arearegions 24 are gallium arsenide. The limited area regions areapproximately circular in the lateral surface and preferably have atransverse thickness t at least as great as their maximum lateraldimension L.

This preferred embodiment can be fabricated by growing on a conventional(100) silicon IC wafer 20 a layer of silicon oxide 22 having a thicknesspreferably in the range from 5 to 100 microns. Conventionalphotolithography can be used with HF etchant to open windows 23, andgallium arsenide having a thickness greater than or approximately equalto the maximum lateral dimension is deposited on the exposed silicon byMBE with a substrate temperature of 570° C. Preferably the thickness ofthe silicon oxide and the gallium arsenide are equal in order to producea co-planar structure as shown in FIG. 2.

FIG. 3 is a schematic cross section of a second embodiment of asemiconductor workpiece provided with limited area regions of low defectdensity. The embodiment is similar to that shown in FIG. 2 except thatthe substrate 20 is provided with one or more limited area mesa regions30 upon which the limited area regions 24 of low defect densitysemiconductor are grown. The mesas are substantially surrounded bytrenches 31.

This embodiment can be fabricated by forming on a silicon substrate analuminum mask which selectively exposes trench regions 31. The maskedsubstrate is then subjected to reactive ion etching to produce trenches31. The aluminum is removed over the mesas, and a silicon oxide layer isdeposited. The silicon oxide over the mesas is selectively removed, andthe gallium arsenide region 24, having a thickness preferably in excessof its maximum lateral dimension, is deposited by CVD at a temperatureof about 600°-700° C. or by MBE at about 550°-650° C. Any GaAs depositedon the oxide covered areas can be removed by dissolving the underlyingsilicon oxide in HF. Finally a planarized insulating layer 22, such assilicon oxide can be applied to the non-mesa areas, resulting in thestructure shown in FIG. 3.

FIG. 4 is a schematic cross section of a third embodiment of theinvention wherein a semiconductor substrate 20 is provided with limitedarea regions 24 of low defect density by providing a plurality ofrelaxed, misfitted buffer layers 42 and 43 between the substrate and thelow defect layer.

In essence the preferred form of the FIG. 4 embodiment is similar to thepreferred form of the FIG. 3 embodiment except that disposed between theupper surface of silicon mesa 30 and limited area gallium arsenideregion 24 is a limited area region 42 of germanium silicon alloy Ge_(x)Si_(1-x) having an upper surface 43 of substantially pure germanium uponwhich gallium arsenide region 24 is grown. The Ge_(x) Si_(1-x) region isgrown as a limited area region having an area in the range between 25and 10,000 square microns. The Ge_(x) Si_(1-x) region 42 has gradient ofincreasing Ge concentration as the region extends from the mesa 30 tothe gallium arsenide layer 24. The advantage of growing this structurein limited area is that a high proportion of threading segments from theinterface with substrate 20 can glide out to the sides of the structure.Thus the Germanium surface 43 presents the gallium arsenide layer 24with a very low defect substrate. If desired, highly effective furtherfiltering can be provided by growing layer 24 in sufficient thickness tin relation to maximum lateral dimension L that threading segments exitthe sides.

The workpiece of FIG. 4 can be prepared by etching trenches 31 to definemesas 30, and depositing Ge_(x) Si_(1-x) onto the mesas by the MBE orCVD processes. The Ge concentration increased linearly with thickness orstep graded at a rate in the range between 5% and 0.1% per one thousandangstroms until the concentration of germanium is substantially 100%.The temperature of growth should be greater than about 600° C. For theCVD process the temperature is preferably about 900° C. and for the MBEprocess, preferably 650°-750° C.

Once the pure germanium concentration is reached, either the GaAs layercan be grown immediately or a Ge buffer layer 43 of about 1000 angstromscan be grown before the GaAs deposition.

Semiconductor workpieces as shown in FIGS. 2, 3 and 4 are highlyadvantageous in that they present upper surfaces of epitaxial regions 24that are substantially free of dislocation defects. While the low defectsurfaces are limited in area, they present areas of 25 to 10,000 squaremicrons that are large enough to permit fabrication of usefuloptoelectronic devices and high speed integrated circuits. Primary usesinclude 1) provision of optical input, optical output and opticalinterconnections to integrated circuits in the substrate; 2) provisionof high speed supplementary circuitry in support of integrated circuitsin the substrate; and 3) provision of high yield areas in low yieldsubstrates.

FIG. 5 schematically illustrates the use of the invention to provideoptical input, optical interconnections and optical output to integratedcircuits in the substrate. Specifically, the substrate 20 is preferablya monolithic silicon substrate containing one or more integratedcircuits 50A and 50B and one or more limited area, low defect galliumarsenide regions 24A, 24B, 24C and 24D.

Integrated circuit 50A is provided with optical input, as from opticalfiber 51A, by forming a photodector 52A on limited area gallium arsenideregion 24A. The optical signal coupled to the photodetector produces anelectrical signal coupled to circuit 50A by conformal metal leads 53.

Integrated circuit 50A is provided with optical output, as to opticalwaveguide 54, by forming a light emitter 55A, such as a LED or laser, onlimited area gallium arsenide region 24B. An electrical signal fromcircuit 50A is coupled to light emitter 55A by metal leads 56. The lightemitter, in turn, produces a modulated optical output signal coupled bywaveguide 54 to a second photodetector 52B formed on limited area region24C. The electrical output of 52B is coupled to a second integratedcircuit 50B by metal leads 57. Thus integrated circuits 50A and 50B areprovided with optical interconnections.

As illustrated, the system can similarly be provided with an opticaloutput as by a second light emitter 55B formed on limited area region24D. An electrical output signal from circuit 50B over leads 53 causesemitter 55B to produce an optical output signal coupled into opticalfiber 51B.

The integrated circuits 50A and 50B can be any of a large number ofknown silicon VLSI circuits useful, for example, in processing serialdigital signals. The structure and fabrication of such circuits is wellknown in the art.

Limited area gallium arsenide regions 24A, 24B, 24C, and 24D can befabricated on substrate 20 after formation of integrated circuits 50Aand 50B without significantly deteriorating the underlying integratedcircuits. MBE at 550°-650° C. is particularly advantageous because ofthe low deposition temperatures. Photodetectors 52A and 52B can beformed on regions 24A and 24C in accordance with one of a variety ofknown methods of forming photodetectors on gallium arsenide substrates.See, for example, the photodetectors disclosed in Smith et al, "A NewInfrared Detector Using Electron Emission From Multiple Quantum Wells,"J. Vac. Sci. Technol. B, Vol. 1, No. 2, April-June 1983 and Levine etal, "New 10 Micron Infrared Detector Using Intersubband Absorption InResonant Tunneling GaAlAs Superlattices," Applied Physics Letters, Vol.50, No. 16, Apr. 20, 1987. Similarly light emitters 55A and 55B can beformed on regions 24B and 24D in accordance with one of a variety ofknown methods for forming LED's or lasers on gallium arsenidesubstrates. See, for example, Windhorn et al, "Al GaAs/GaAs Laser Diodeson Si", Applied Physics Letters, Vol. 47, p. 1031 (1985); Ettenburg,"Continuous Low Threshold AlGaAs/GaAs Laser", Applied Physics Letters,Vol. 27, p. 652 (1975), or the previously cited Choi et al articles.Waveguide 54 can be polymer, silicon oxide, or glass. Preferably it is aphosphosilicate glass waveguide such as described in Henry, "RecentAdvances in Integrated Optics on Silicon", Proceedings of Eighth AnnualEuropean Fiber Optic Communications and Local Area Networks Conference,Jun. 27-29, 1990.

FIG. 6 schematically illustrates the use of the invention to provide asupplementary high-speed circuit to an integrated circuit in thesubstrate. Here, as above, the substrate 20 is preferably a monolithicsilicon substrate containing one or more integrated circuits 50 and oneor more limited area, low defect density gallium arsenide regions 24.The primary difference between this embodiment and that of FIG. 5 isthat in FIG. 6 the regions 24 is sufficiently large to contain a smallintegrated circuit 60 rather than only a device. For example, a limitedarea region 24 having dimensions 50×50×50 microns is sufficiently largeto contain the GaAs MESFET Circuit described in Shichijo, et al,"Co-Integration of GaAs MESFET and Si CMOS Circuits", IEEE ElectronDevice Letters, Vol. 9, No. 9, September 1988. The silicon integratedcircuit 50 can be CMOS inverter stages. The combination can form a ringoscillator. The advantage of using limited area regions 24 in accordancewith the invention is lower defects in the gallium arsenide withresulting higher yield and improved performance.

FIG. 7 schematically illustrates the use of the invention to providearrays of low defect density regions integrally formed on a high defectdensity substrate. Here substrate 20 is a monolithic crystallinesubstrate having a level of defect density sufficiently large topreclude high yield or to limit desired quality of devices formed on thesubstrate. For example, substrate 20 can be gallium arsenide having adefect density in excess of about 10³ cm⁻².

As a preliminary step, substrate 20 is etched, as by reactive ionetching, to form a sequence of pits 70 and mesas 71. Gallium arsenide isdeposited as by MBE to form low defect regions 24A on the mesas 30, andlow defect regions 24B can be simultaneously formed in the pits. Bygrowing the gallium arsenide sufficiently thick as compared with itsmaximum lateral dimension, the regions 24A and 24B are provided withupper surfaces substantially free of defects. The resulting structurecan be used to fabricate on the upper surfaces of regions 24A and 24B,arrays of devices such as photodetectors and lasers, having reduceddefects and resulting higher yield and performance.

It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can be readily devised inaccordance with these principles by those skilled in the art withoutdeparting from the spirit and scope of the invention.

I claim:
 1. A semiconductor device having one or more limited arearegions with reduced defect density semiconductor surfaces comprising:amonocrystalline semiconductor substrate; disposed on said substrate oneor more epitaxially grown limited area regions of semiconductor materialhaving an area in the range between 25 and 10,000 square microns, amaximum lateral dimension L and a thickness in excess of about ##EQU1##2. A semiconductor device in accordance with claim 1 wherein:saidsubstrate is provided with one or more pit regions of depthsubstantially equal to the thickness of said limited area regions; andsaid limited area regions are disposed in said pit regions.
 3. Asemiconductor device in accordance with claim 1 wherein insulatingmaterial is disposed on said substrate around sides of one or more ofsaid limited area regions.
 4. A semiconductor device in accordance withclaim 1 wherein said substrate is comprised of a first crystallinematerial, and said limited area regions are comprised of a secondcrystalline material, said first and second materials having a latticemismatch in excess of about 0.2%.
 5. A semiconductor device inaccordance with claim 1 wherein said substrate comprises silicon andsaid limited area regions comprise gallium arsenide.
 6. A semiconductordevice in accordance with claim 1 wherein said substrate has a densityof dislocation defects in excess of about 10³ per cm² and said limitedarea regions are comprised of the same kind of semiconductor material assaid substrate.
 7. A semiconductor device in accordance with claim 1wherein said substrate comprises gallium arsenide and said limited arearegions comprise gallium arsenide.
 8. A semiconductor device inaccordance with claim 1 wherein said limited area regions comprise oneor more photodetector means for receiving an optical input signal andproducing an electrical signal;said substrate comprises integratedcircuit means for receiving said electrical signal and for producing anelectrical output signal; and said limited area regions further compriselight emitter means responsive to said electrical output signal fromsaid integrated circuit means for producing an optical output signal. 9.A semiconductor device in accordance with claim 1 wherein said limitedarea regions comprise integrated circuit means responsive to an inputsignal for digitally processing said input signal and producing anelectrical output indicative of said processed input signal; andsaidsubstrate comprises integrated circuit means responsive to saidelectrical output for digitally processing said output.